Thermoelectric energy storage system and method for storing thermoelectric energy

ABSTRACT

A system and method are provided for thermoelectric energy storage. A thermoelectric energy storage system having at least one hot storage unit is provided. In an exemplary embodiment, each hot storage unit includes a hot tank and a cold tank connected via a heat exchanger and containing a thermal storage medium. The thermoelectric energy storage system also includes a working fluid circuit for circulating working fluid through each heat exchanger for heat transfer with the thermal storage medium. Improved roundtrip efficiency is achieved by minimizing the temperature difference between the working fluid and the thermal storage medium in each heat exchanger during heat transfer. Exemplary embodiments realize this improved roundtrip efficiency through modification of thermal storage media parameters.

RELATED APPLICATIONS

This application claims priority as a continuation application under 35U.S.C. §120 to PCT/EP2009/058475, which was filed as an InternationalApplication on Jul. 6, 2009, designating the U.S., and which claimspriority to European Application 08160520.6 filed in Europe on Jul. 16,2008. The entire contents of these applications are hereby incorporatedby reference in their entireties.

FIELD

The present disclosure relates generally to the storage of electricenergy. More particularly, the present disclosure relates to a systemand method for storing electric energy in the form of thermal energy ina thermal energy storage.

BACKGROUND INFORMATION

Base load generators such as nuclear power plants and generators withstochastic, intermittent energy sources, such as wind turbines and solarpanels, generate excess electrical power during times of low powerdemand. Large-scale electrical energy storage systems are a means ofdiverting this excess energy to times of peak demand and balancingoverall electricity generation and consumption.

In EP1577548, the applicant described the idea of a thermoelectricenergy storage (TEES) system. A TEES converts excess electricity toheat, stores the heat, and converts the heat back to electricity, whennecessary. Such an energy storage system is robust, compact, siteindependent and is suited to the storage of electrical energy in largeamounts. Thermal energy can be stored in the form of sensible heat via achange in temperature or in the form of latent heat via a change ofphase or a combination of both. The storage medium for the sensible heatcan be a solid, liquid, or a gas. The storage medium for the latent heatoccurs via a change of phase and can involve any of these phases or acombination of the phases in series or in parallel.

All electric energy storage technologies inherently have a limitedround-trip efficiency. Thus, for every unit of electrical energy used tocharge the storage, only a certain percentage is recovered as electricalenergy upon discharge. The rest of the electrical energy is lost. If,for example, the heat being stored in a TEES system is provided throughresistor heaters, it has approximately 40% round-trip efficiency. Theefficiency of thermoelectric energy storage is limited for variousreasons rooted in the second law of thermodynamics. Firstly, theconversion of heat to mechanical work is limited to the Carnotefficiency. Secondly, the coefficient of performance of any heat pumpdeclines with increased temperature difference between the input leveland the output level. Thirdly, any heat flow from a working fluid to athermal storage and vice versa requires a temperature difference inorder to happen. This fact inevitably degrades the temperature level andthus the capability of the heat to do work.

It is noted that many industrial processes involve provision of thermalenergy and storage of the thermal energy. Examples are refrigerationdevices, heat pumps, air conditioning and the process industry. In solarthermal power plants, heat is provided, possibly stored, and convertedto electrical energy. However, all these applications are distinct fromTEES systems because they are not concerned with heat for the exclusivepurpose of storing electricity.

It is known in the art that heat can be provided to the thermal storageunit through a heat pump. For example, a Stirling machine (forreference, see U.S. Pat. No. 3,080,706, column 2, lines 22-30). Also, WO2007/134466 discloses a TEES system having an integrated heat pump.

A heat pump requires work to move thermal energy from a cold source to awarmer heat sink. Since the amount of energy deposited at the hot sideis greater than the work required by an amount equal to the energy takenfrom the cold side, a heat pump will “multiply” the heat as compared toresistive heat generation. The ratio of heat output to work input iscalled a coefficient of performance, and the ratio is a value largerthan one. In this way, the use of a heat pump will increase theround-trip efficiency of a thermoelectric energy storage system. Theround-trip efficiency is the amount of electricity provided from thestorage divided by the amount of electricity provided to the storage.

U.S. Pat. No. 4,089,744 discloses a method of thermal energy storage bymeans of reversible heat pumping. Excess electrical output is stored inthe form of sensible heat by using it to raise the temperature level ofa heat storage fluid. In this scheme, the source of low level heat isstored hot water, which also serves as the working fluid in the heatpump and the turbine cycles. A thermodynamic analysis, such as the typeof analysis shown in FIG. 6 of the '744 patent, shows that theefficiency of schemes equivalent to that of U.S. Pat. No. 4,089,744 islimited to about 50%.

In view of this background, exemplary embodiments of the presentdisclosure provide an efficient thermoelectric energy storage having around-trip efficiency of, for example, greater than 55%.

SUMMARY

An exemplary embodiment provides a thermoelectric energy storage systemfor providing thermal energy to a thermodynamic machine for generatingelectricity. The exemplary system includes a heat exchanger, and a hotstorage unit which is connected to the heat exchanger and which containsa thermal storage medium. The exemplary system also includes a workingfluid circuit configured to circulate a working fluid through the heatexchanger for heat transfer with the thermal storage medium contained inthe hot storage unit. The working fluid circuit is also configured tominimize a temperature difference between the working fluid and thethermal storage medium in the hot storage unit during heat transfer.

An exemplary embodiment also provides a method for storingthermoelectric energy in a thermoelectric energy storage system. Theexemplary method includes charging a hot storage unit by providing heatvia a heat exchanger to a thermal storage medium by compressing aworking fluid. The exemplary method also includes discharging the hotstorage unit by expanding the working fluid heated via the heatexchanger from the thermal storage medium through a thermodynamicmachine. In addition, the exemplary method includes modifying thermalstorage media parameters to ensure that a temperature difference betweenthe working fluid and the thermal storage medium is minimized duringcharging and discharging.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional refinements, advantages and features of the presentdisclosure are described in more detail below with reference toexemplary embodiments illustrated in the drawings, in which:

FIG. 1 shows an excerpt of a Substation Automation (SA) system, and asimplified schematic diagram of an exemplary thermoelectric energystorage system according to an embodiment of the present disclosure;

FIG. 2 is an enthalpy-pressure diagram of the heat pump cycle and theturbine cycle in an exemplary TEES system according to an embodiment ofthe present disclosure;

FIG. 3 is a schematic illustration of a cross-section through a heatpump cycle portion of an exemplary TEES system according to anembodiment of the present disclosure;

FIG. 4 is a schematic illustration of a cross-section through a turbinecycle portion of an exemplary TEES system according to an embodiment ofthe present disclosure;

FIGS. 5 a-5 f depict simplified enthalpy-temperature diagrams of theworking fluids and thermal storage fluids in exemplary heat exchangersduring charging and discharging, according to an embodiment of thepresent disclosure;

FIG. 6 shows an enthalpy-temperature diagram of the heat transfer fromthe cycles in an exemplary TEES system according to an embodiment of thepresent disclosure; and

FIG. 7 shows an enthalpy-temperature diagram of the heat transfer fromthe cycles in an optimized scenario in an exemplary TEES systemaccording to an embodiment of the present disclosure.

For consistency, the same reference numerals are used to denote similarelements or similarly functioning elements illustrated throughout thedrawings.

DETAILED DESCRIPTION

Exemplary embodiments of the present disclosure provide a thermoelectricenergy storage system for converting electrical energy into thermalenergy to be stored and converted back to electrical energy with animproved round-trip efficiency. An exemplary embodiment provides athermoelectric energy storage system for providing thermal energy to athermodynamic machine for generating electricity. Another exemplaryembodiment provides a method for storing thermoelectric energy in athermoelectric energy storage system.

According to an exemplary embodiment of the present disclosure, athermoelectric energy storage system is provided which includes a hotstorage unit which is in connection with a heat exchanger and whichcontains a thermal storage medium. The exemplary system also includes aworking fluid circuit for circulating a working fluid through the heatexchanger for heat transfer with the thermal storage medium. Thetemperature difference between the working fluid and the thermal storagemedium in the hot storage unit is minimized during heat transfer.

When the thermoelectric energy storage system is in a charging (or “heatpump”) cycle, the thermodynamic machine includes a turbine, and when thethermoelectric energy storage system is in a discharging (or “turbine”)cycle, the thermodynamic machine includes a compressor.

According to an exemplary embodiment, the hot storage unit can includeat least two hot storage units, where each hot storage unit is inconnection with a respective heat exchanger and contains a thermalstorage medium.

In accordance with an exemplary embodiment, the heat exchanger or heatexchangers are common to both the charging and discharging cycles.However, it is also possible that there are separate heat exchangers forthe charging and discharging cycles. Two or more heat exchangersutilized in series can be connected hydraulically, for example.

The thermal storage medium may be a liquid, and a flow rate of thethermal storage medium may be modified such that the temperaturedifference between the working fluid and the thermal storage medium ineach hot storage unit is minimized during heat transfer.

According to an exemplary embodiment, the thermal storage medium may bea solid or a liquid. The particular exemplary embodiment illustrated inFIGS. 3 and 4 of the accompanying description shows an example in whichthe thermal storage medium is a liquid.

In accordance with an exemplary embodiment, a single working fluidcircuit containing a single type of working fluid can be utilized forboth the charging and discharging cycles. However, it is also possiblefor there to be separate working fluid circuits for the charging anddischarging cycles. Further, each separate working fluid circuit maycontain a different type of working fluid.

According to an exemplary embodiment, the temperature of the thermalstorage medium at entry and exit points of each connected heat exchangercan be modified such that the temperature difference between the workingfluid and the thermal storage medium in each hot storage unit isminimized during heat transfer.

In accordance with an exemplary embodiment, at least one of the hotstorage units may contain a different type of thermal storage mediumsuch that the temperature difference between the working fluid and thethermal storage medium in each hot storage unit is minimized during heattransfer.

In accordance with an exemplary embodiment, the hot storage unit orunits include a thermal storage medium for sensible heat storage and aphase change storage medium for latent heat storage, which are arrangedsuch that the temperature difference between the working fluid and thethermal storage medium in each heat exchanger unit is minimized duringheat transfer.

In accordance with an exemplary embodiment, the temperature differencebetween the working fluid and the thermal storage medium in each hotstorage unit can be less than 50° C. during heat transfer, for example.

In accordance with an exemplary embodiment of the present disclosure, amethod is provided for storing thermoelectric energy in a thermoelectricenergy storage system. The exemplary method includes charging a hotstorage unit by providing heat via a heat exchanger to a thermal storagemedium by compressing a working fluid. The exemplary method alsoincludes discharging the hot storage unit by expanding the working fluidheated via the heat exchanger from the thermal storage medium through athermodynamic machine. In addition, the exemplary method includesmodifying the thermal storage media parameters to ensure the temperaturedifference between the working fluid and the thermal storage medium isminimized during charging and discharging.

In accordance with an exemplary embodiment, the step of modifying thethermal storage media parameters can include modifying the flow rate ofthe thermal storage medium.

In accordance with an exemplary embodiment, the step of modifying thethermal storage media parameters can include modifying the initialtemperature and final temperature of the thermal storage medium.

In accordance with an exemplary embodiment, the step of modifying thethermal storage media parameters can include modifying the type ofthermal storage medium.

FIG. 1 depicts a schematic diagram of an exemplary TEES system 10 inaccordance with an embodiment of the present disclosure. The TEES system10 includes a hot storage 12 and a cold storage 14 which are coupled toeach other by means of a heat pump cycle system 16 and a turbine cyclesystem 18. The hot storage 12 contains a thermal storage medium.According to an exemplary embodiment, the cold storage 14 can be a heatsink, for example. Both the heat pump cycle and the turbine cyclecontain a working fluid.

The heat pump cycle system 16 includes, in the flow direction of theworking fluid, an evaporator 20, a compressor train 22, a heat exchanger24, and an expansion valve 26. The turbine cycle system 18 includes, inthe flow direction of the working fluid, a feed pump 28, a heatexchanger 30, a turbine 32, and a condenser 34. The heat exchangers 24,30 in both the heat pump cycle system 16 and the turbine cycle system18, respectively, are arranged to exchange heat with the hot storage 12.The evaporator 20 and the condenser 34 in the heat pump cycle system 16and the turbine cycle system 18, respectively, are arranged to exchangeheat with the cold storage 14.

The cold storage 14 is a heat reservoir at any temperature lower thanthe hot storage temperature. However, the cold storage temperature maybe higher or lower than the ambient temperature. For example, the coldstorage may be another heat sink such as cooling water or air from theambient. In an alternative embodiment, the turbine and compressor trains22, 32 may be thermodynamic machines based on positive displacement suchas reciprocating or rotary expanders or compressors.

The compressor train 22 may include one or more individual compressorswith possible intercooling. The turbine 32 may include one or moreindividual turbines with possible reheating. Similarly, the evaporator20, the condenser 34, the feed pump 28 and the expansion valve 26 mayinclude one or multiple units.

In operation, the working fluid flows around the TEES system 10 in thefollowing manner. The working fluid in the compressor 22 is initially invapor form, and surplus electrical energy is utilized to compress andheat the working fluid. The working fluid is fed through the heatexchanger 24 where the working fluid discards heat into the storagemedium of the hot storage 12. The compressed working fluid exits theheat exchanger 24 and enters the expansion valve 26. Here, the workingfluid is expanded to the lower pressure of the evaporator 20. Theworking fluid flows from the expansion valve into the evaporator 20where the working fluid is heated to evaporation. This is realized usingavailable heat from the cold storage 14.

In the condenser 34, working fluid is condensed by exchanging heat withthe cold storage 14. The condensed working fluid exits the condenser 34via an outlet and is pumped into the heat exchanger 30 at the hotstorage 12 via the feed pump 28. Here, the working fluid is heated,evaporated, and overheated from the stored heat from the storage mediumin the hot storage 12. The working fluid exits the heat exchanger 30 andenters the turbine 32 where the working fluid is expanded to therebycause the turbine to generate electrical energy.

The expansion valve 26, the evaporator 20, and the compressor 22 are inoperation during a period of charging, or the “heat pump cycle”.Similarly, the turbine 32, the condenser 34 and the feed pump 28 are inoperation during a period of discharging, or the “turbine cycle”. Thehot storage 12 is in operation at all times, i.e., during charging,storage, and discharging. These two cycles can be clearly shown in anenthalpy-pressure diagram, such as FIG. 2.

The solid-line cycle shown in FIG. 2 represents the heat pump cycle thatis charging the hot storage, and the heat pump cycle follows acounter-clockwise direction as indicated by the arrows. The workingfluid is assumed to be water for this exemplary embodiment. The heatpump cycle starts in the evaporator at point A where steam is evaporatedto form vapor using heat from the cold storage (transition A→B1 in FIG.2). In the next stage of the heat pump cycle, the vapor is compressedutilizing electrical energy in two stages from point B1 to C1 and B2 toC2. Where compression occurs in two stages, this is a consequence of thecompressor train 22 including two individual units. In between these twocompression stages, the working fluid is cooled from point C1 to B2. Thehot, compressed, overheated vapor exits the compression train 22 atpoint C2 where it is cooled down to the saturation temperature at D1,condensed at D2, and further cooled down to point D3. This cooling downand condensation is realized by transferring the heat from the workingfluid into the hot storage 12 thereby storing the heat energy. Thecooled working fluid is returned to its initial low pressure state atpoint A via the expansion valve 26.

The dotted-line cycle shown in FIG. 2 represents the Rankine turbinecycle that is discharging the hot storage, and the cycle follows aclockwise direction as indicated by the arrows. The Rankine turbinecycle starts at point E, where the pump 28 is utilized to pump theworking fluid in its liquid state from point E to F1. Next, from pointF1 to point G, the working fluid receives the heat from the thermalstorage medium. In detail, the heat is transferred from the thermalstorage medium to the working fluid causing the working fluid to heat upat F2, to boil at F3, and attain a certain degree of superheat at G. Thesuperheated working fluid vapor at point G is expanded down to point Hin a mechanical device such as a turbine to generate electricity.Following the expansion, the working fluid enters the condenser 34 whereit is condensed to its initial state at point E by exchanging heat withthe cold storage 14.

The roundtrip efficiency of the complete energy storage process, that isthe heat pump cycle and the Rankine turbine cycle, is calculated in thefollowing manner; the work provided by the turbine expansion divided bythe work used in the heat pump compressor:

(h_(G)−h_(H))/(h_(C2)−h_(B2)+h_(C1)−h_(B1)),

where the letter h denotes the enthalpy of the corresponding point. Forthe exemplary conditions depicted in FIG. 2, the roundtrip efficiency is50.8%. It is not possible from the enthalpy-pressure diagram alone tojudge if this is a particularly efficient TEES system, or how it couldbe improved in efficiency.

With reference to the exemplary TEES system illustrated in FIG. 1, theheat exchanger 24 in the heat pump cycle components 16 and the heatexchanger 30 in the turbine cycle components 18 may include severalindividual heat exchangers arranged in series, as illustrated in FIGS. 3and 4, respectively.

FIG. 3 depicts a simplified schematic diagram of the heat pump cyclecomponents 16 in a thermoelectric energy storage system 10 according toan exemplary embodiment of the present disclosure. Here, threeindividual hot storage units x, y, z are arranged in series. Each hotstorage unit x, y, z comprises a heat exchanger 36, 38, 40 in connectionwith a storage tank pair 42, 44, 46, respectively. Each storage tankpair comprises a cold tank and a hot tank wherein the flow of thethermal storage medium is from the cold tank to the hot tank via theassociated heat exchanger. The three hot storage units in FIG. 3 aredenoted x, y and z from left to right in the diagram. In the presentembodiment, the heat exchangers are counterflow heat exchangers, and theworking fluid of the cycle is water.

In operation, the heat pump cycle components 16 of FIG. 3 performessentially in a similar manner as the heat pump cycle components 16 ofthe TEES system 10 described with respect to FIGS. 1 and 2. In addition,the working fluid flows through the additional two separate heatexchangers. In the exemplary embodiment shown in FIG. 3, in thedirection of flow of the working fluid, the initial and finaltemperatures of the working fluid as it passes through heat exchanger 40are 510° C. and 270° C., through heat exchanger 38 are 270° C. and 270°C., and through heat exchanger 36 are 270° C. and 100° C. Thus, anoverall temperature drop of 410° C. is achieved.

The characteristics of the working fluid (shown as a solid line) andthermal storage medium (shown as a dashed line) of each of the threeheat exchangers 36, 38, 40 and associated storage tank pair 42, 44, 46during charging are shown in FIG. 5 in the enthalpy-temperature graphsa), b) and c), respectively. The temperature of the thermal storagemedium in each stage is increasing, while the temperature of the workingfluid decreases only in stages a) and c).

FIG. 4 depicts a simplified schematic diagram of the turbine cyclecomponents 18 in a thermoelectric energy storage system 10 according toan exemplary embodiment of the present disclosure. Here, the arrangementof three individual hot storage units x, y, z, arranged in series, arethe same units shown in FIG. 3. Again, each storage tank pair 42, 44, 46includes a hot tank and a cold tank. However, in the exemplaryembodiment of FIG. 4, the flow of the thermal storage medium is from thehot tank to the cold tank via the heat exchanger.

In operation, the turbine cycle components 18 of FIG. 4 performessentially in a similar manner as the turbine cycle components of theTEES system described with respect to FIGS. 1 and 2. In addition, theworking fluid flows through the additional two separate heat exchangers.In the exemplary embodiment shown in FIG. 4, in the direction of flow ofthe working fluid, the initial and final temperatures of the workingfluid as it passes through heat exchanger 36 are 80° C. and 240° C.,through heat exchanger 38 are 240° C. and 240° C., and through heatexchanger 40 are 240° C. and 490° C. Thus, an overall temperatureincrease of 410° C. is achieved.

When the heat pump cycle components 16 are in operation, then theworking fluid conduit for the heat pump cycle is coupled to the hotstorage units x, y, z. When the turbine pump cycle components 18 are inoperation, then the working fluid conduit for the turbine cycle coupledto the hot storage units x, y, z, instead. In this way, the turbinecycle obtains thermal energy from the hot storage units that wasdeposited by the heat pump cycle.

The characteristics of the working fluid (shown as a solid line) andthermal storage medium (shown as a dashed line) of each of the threeheat exchangers 36, 38, 40 and associated storage tank pairs 42, 44, 46during discharging are shown in FIG. 5 in the enthalpy-temperaturegraphs d), e) and f), respectively. The temperature of the thermalstorage medium in each stage is decreasing, while the temperature of theworking fluid increases only in stages d) and f).

FIG. 6 shows the isobars, e.g., lines of constant pressure, from FIG. 5a)-f) on a single temperature-enthalpy graph for a particular exemplaryembodiment of the present disclosure. Further, the capital letters usedare consistent with FIG. 2. Thus, FIG. 6 illustrates the heat transferprocess at the three separate hot storage units x, y, z during thecharging and discharging of the TEES system 10.

The solid line isobars C2 to D3 represent the heat pump cycle, thedotted line isobars F1 to G represent the Rankine turbine cycle, and thedashed line isobars X1 to X2, Y1 to Y2, Z1 to Z2 represent the thermalstorage media in the three hot storage units x, y, z, respectively.

Heat can only flow from a higher to a lower temperature. Consequently,the characteristic isobars for the working fluid during cooling in theheat pump cycle have to be above the characteristic isobars for thethermal storage media, which in turn have to be above the characteristicisobars for the working fluid during heating in the turbine cycle. Theslope of these characteristic isobars is defined by the product of themassflow (kg/s) and heat capacity (J/kg/K) of each thermal storagemedium relative to the massflow of the working fluid. This product isdifferent for each of the three heat transfer subsections:heating/cooling of liquid water in hot storage unit x;boiling/condensation in hot storage unit y; and providing/extractingheat to the supersaturation region in hot storage unit z.

The temperature profiles are stationary in time due to the sensible heatstorage in the thermal storage media. Thus, while the volume of thermalstorage media in each heat exchanger remains constant, the volume of hotand cold thermal storage media stored in the hot and cold tanks changes.Also, the temperature distribution in the heat exchangers remainsconstant.

Importantly, exemplary embodiments of the present disclosure determinethat the smaller the average temperature difference between the workingfluid and the heat storage media during heat transfer, the greater theefficiency of the TEES system. In an enthalpy-temperature graph, thisfeature is observed as a relatively closer positioning of thecharacteristic isobars of the charging and discharging cycles, as shownin FIG. 7.

Exemplary embodiments of the present disclosure determine that thethermal storage media may be the same or a different fluid in each hotstorage unit x, y and z. Further, exemplary embodiments of the presentdisclosure determine that the thermal storage media may be at adifferent temperature in each hot storage unit x, y and z. Also, theflow-rate of the thermal storage media within each hot storage unit maydiffer. Specifically, in order to achieve an optimized roundtripefficiency of the TEES system, various combinations of the thermalstorage media, the initial and final temperature of the thermal storagemedia and the thermal storage media flow-rates may be utilized.

In the improved efficiency scenario illustrated in FIG. 7, the flow-rateof the thermal storage medium through heat exchanger 38 of hot storageunit y is increased by a factor of three in comparison with the scenarioin FIG. 6. (It should be noted that the flow rate in heat exchanger 38,in FIG. 6, was set to an arbitrary rate that was relatively larger thanthe flow rate in heat exchangers 36 and 40, but the flow rate was notoptimized as in FIG. 7.) A decrease in average temperature differencesbetween the thermal storage medium and the working fluid during heattransfer in heat exchanger 38 of hot storage unit y can be noted.Consequently, a resultant TEES system design has a higher saturationtemperature in heat exchanger 38 in the turbine cycle than before(denoted as F2′ and F3′ in FIG. 7 in comparison with F2 and F3 in FIG.6). This equates to a temperature of 230° C. in FIG. 7, in comparisonwith 200° C. in FIG. 6. Consequently, the roundtrip efficiency of theTEES system in the embodiment of FIG. 7 is 61.1% in comparison to anefficiency of 50.8% in FIG. 2.

In others words, exemplary embodiments of the present disclosure requirethe temperature difference between the working fluid of the heat pumpcycle and the heat storage media, as well as the temperature differencebetween the working fluid of the turbine cycle and the heat storagemedia to be relatively small (for example, smaller than 50° C. onaverage). This is achieved through modification of certain TEESparameters as specified above.

In an accordance with an exemplary embodiment of the present disclosure,the three thermal storage media are fluids. For example, these may bethree different liquid sensible heat storage media such as water, oil,or molten salts. Also, in an accordance with an exemplary embodiment ofthe present disclosure, the heat exchangers are counterflow heatexchangers, having a minimal approach temperature 10 K (e.g., theminimal temperature difference between the two fluids exchanging heat is10 K) and the expansion device can be a thermostatic expansion valve.

In accordance with an exemplary embodiment, the heat at theboiling/condensation heat exchanger 38 is transferred to the latent heatof a phase transition of a storage medium enabling an even closer matchof the temperature profiles in the boiling/condensation region. Anexemplary embodiment uses steam as the working fluid for both the heatpump cycle and the turbine cycle.

In an alternative exemplary embodiment, there is no cold storagereservoir, but the evaporator and condenser instead use heat from theambient as an (infinitely large) reservoir for the cold side of the heatpump cycle and the turbine cycle. The cold storage of FIG. 1, which is asecond heat storage reservoir, has latent heat storage at temperaturesaround 100° C. at the cold side of the heat pump cycle and the turbinecycle. Because of the temperature dependence of the saturation pressureof working fluids such as water, such an additional heat storagereservoir may result in greater economy with respect to the compressorand the turbine. It is envisaged that this economy would more thancompensate for the additional cost for this reservoir at moderately longstorage times.

The skilled person will appreciate that the exemplary TEES system, asillustrated in FIGS. 1, 3 and 4, may be realized in several differentways. For example, the hot storage can be constituted by:

-   -   A solid structure with embedded heat exchangers equipped with        appropriate means of handling the expansion-contraction of the        storage medium with changing temperatures.    -   A two-tank molten salt storage system with heat exchangers        between the tanks and flow of molten salt from the cold to the        hot tank during charging and from the hot to the cold tank        during discharging periods.    -   A multiple-hot-tank multiple-cold-tank molten salt and liquid        heat storage media cascaded at different temperatures between        the evaporator operating temperature and the temperature of the        heat pump working fluid at the exit of the compression        processes.    -   A phase change material with a suitable phase change temperature        below the condensation temperature of the heat pump working        fluid at the high operating pressure and above the boiling point        of the turbine cycle working fluid at the high operating        pressure.

Any combination of the above mentioned thermal storage options in seriesand in parallel.

-   -   Two, three (as shown in FIGS. 3 and 4), four or more hot storage        units in the hot storage.

The skilled person will appreciate that the condenser and the evaporatorin the exemplary TEES system may be replaced with a multi-purpose heatexchange device that can assume both roles, since the evaporation forthe heat pump cycle and the condensation for the turbine cycle will becarried out in different periods. Similarly, the turbine and thecompressor roles can be carried out by the same machinery, referred toherein as a thermodynamic machine, capable of achieving both tasks.

In the exemplary embodiments described above, The working fluid for iswater, due, in part, to the higher efficiencies of a water-based heatpump cycle and turbine cycle, and the amiable properties of water as aworking fluid, e.g., no global warming potential, no ozone depletionpotential, no health hazards, etc. However, the present disclosure isnot limited thereto. For the operation of the present disclosure atambient temperatures below the freezing point of water, a commercialrefrigerant can be chosen as the heat pump working fluid, or a secondbottoming heat pump cycle can be cascaded with the water-based cycle toprovide the heat of evaporation, for example.

It will be appreciated by those skilled in the art that the presentinvention can be embodied in other specific forms without departing fromthe spirit or essential characteristics thereof. The presently disclosedembodiments are therefore considered in all respects to be illustrativeand not restricted. The scope of the invention is indicated by theappended claims rather than the foregoing description and all changesthat come within the meaning and range and equivalence thereof areintended to be embraced therein.

1. A thermoelectric energy storage system for providing thermal energyto a thermodynamic machine for generating electricity, the systemcomprising: a heat exchanger; a hot storage unit which is connected tothe heat exchanger and which contains a thermal storage medium; and aworking fluid circuit configured to circulate a working fluid throughthe heat exchanger for heat transfer with the thermal storage mediumcontained in the hot storage unit, and wherein working fluid circuit isconfigured to minimize a temperature difference between the workingfluid and the thermal storage medium in the hot storage unit during heattransfer.
 2. The system according to claim 1, wherein the hot storageunit comprises at least two hot storage units, wherein each of the atleast two hot storage units is connected to a respective heat exchangerand contains a thermal storage medium.
 3. The system according to claim1, wherein the thermal storage medium is a liquid, and the working fluidcircuit is configured to modify a flow rate of the thermal storagemedium such that the temperature difference between the working fluidand the thermal storage medium in the hot storage unit is minimizedduring heat transfer.
 4. The system according to claim 1, wherein theworking fluid circuit is configured to modify the temperature of thethermal storage medium at entry and exit points of the heat exchangersuch that the temperature difference between the working fluid and thethermal storage medium in the hot storage unit is minimized during heattransfer.
 5. The system according to claim 2, wherein at least one ofthe hot storage units contains a different type of thermal storagemedium such that the temperature difference between the working fluidand the thermal storage medium in each hot storage unit is minimizedduring heat transfer.
 6. The system according to claim 1, wherein thetemperature difference between the working fluid and the thermal storagemedium in the hot storage unit is less than 50° C. during heat transfer.7. A method for storing thermoelectric energy in a thermoelectric energystorage system, comprising; charging a hot storage unit by providingheat via a heat exchanger to a thermal storage medium by compressing aworking fluid; discharging the hot storage unit by expanding the workingfluid heated via the heat exchanger from the thermal storage mediumthrough a thermodynamic machine; and modifying thermal storage mediaparameters to ensure that a temperature difference between the workingfluid and the thermal storage medium is minimized during charging anddischarging.
 8. The method according to claim 7, wherein the step ofmodifying the thermal storage media parameters comprises modifying theflow rate of the thermal storage medium.
 9. The method according toclaim 7, wherein the step of modifying the thermal storage mediaparameters comprises modifying an initial temperature and finaltemperature of the thermal storage medium.
 10. The method according toclaim 7, wherein the step of modifying the thermal storage mediaparameters comprises modifying the type of thermal storage medium. 11.The system according to claim 2, wherein the thermal storage medium is aliquid, and the working fluid circuit is configured to modify a flowrate of the thermal storage medium such that the temperature differencebetween the working fluid and the thermal storage medium in each hotstorage unit is minimized during heat transfer.
 12. The system accordingto claim 11, wherein at least one of the hot storage units contains adifferent type of thermal storage medium such that the temperaturedifference between the working fluid and the thermal storage medium ineach hot storage unit is minimized during heat transfer.
 13. The systemaccording to claim 2, wherein the working fluid circuit is configured tomodify the temperature of the thermal storage medium at entry and exitpoints of each connected heat exchanger such that the temperaturedifference between the working fluid and the thermal storage medium ineach hot storage unit is minimized during heat transfer.
 14. The systemaccording to claim 13, wherein at least one of the hot storage unitscontains a different type of thermal storage medium such that thetemperature difference between the working fluid and the thermal storagemedium in each hot storage unit is minimized during heat transfer. 15.The system according to claim 3, wherein the working fluid circuit isconfigured to modify the temperature of the thermal storage medium atentry and exit points of each connected heat exchanger such that thetemperature difference between the working fluid and the thermal storagemedium in each hot storage unit is minimized during heat transfer. 16.The system according to claim 15, wherein the temperature differencebetween the working fluid and the thermal storage medium in each hotstorage unit is less than 50° C. during heat transfer.
 17. The systemaccording to claim 2, wherein the temperature difference between theworking fluid and the thermal storage medium in each hot storage unit isless than 50° C. during heat transfer.
 18. The method according to claim8, wherein the step of modifying the thermal storage media parameterscomprises modifying an initial temperature and final temperature of thethermal storage medium.
 19. The method according to claim 8, wherein thestep of modifying the thermal storage media parameters comprisesmodifying the type of thermal storage medium.
 20. The method accordingto claim 9, wherein the step of modifying the thermal storage mediaparameters comprises modifying the type of thermal storage medium.